During normal operation of a rotating storage media device, a read/write head senses servo signals stored on a disk while the head is located over the disk surface. A servo controller interprets the servo signals, and uses these servo signals to adjust the head's position relative to the disk surface. The servo controller moves the head, either to maintain a desired head position or to travel to a new position, by moving an actuator arm whose tip is secured to the head.
During certain situations, however, servo signals are not available to guide or position the head. In one instance, during ramp load or unload operation, the head is not over the region of the disk surface containing servo data. In another instance, during head retract up a ramp after a power failure, the servo controller is not running. Consequently, guidance of the head to and from a ramp cannot be conducted using servo signals.
To overcome this problem, various methods have been used to attempt to estimate head position by analyzing certain electrical characteristics of an actuator's voice coil motor (VCM). A VCM, which is used to position the actuator arm, generally includes a wound conductive coil (called a voice coil, or actuator coil) secured to the actuator arm, and one or more magnets. The coil is positioned within the magnetic field of the magnets. Applying a current through the voice coil creates a magnetic force that moves the actuator coil (and thus, the actuator arm and the head) relative to the magnet(s).
Estimates of voice coil velocity are used to estimate the position of the voice coil, the actuator arm and the head. Methods for estimating the velocity of the voice coil (and thereby, of the actuator arm and the head) typically rely on accurate determinations of the back electromagnetic field voltage (back EMF voltage, or simply VBEMF) present across the voice coil, which is due to the coil's motion through the field of the magnets. More specifically, since the VBEMF is proportional to the voice coil's angular velocity in the ratio of a known constant, it can be used to determine the velocity of the voice coil. For example, the angular velocity of the voice coil can be determined using the following equation:
                    ω        =                              1                          K              T                                ⁢                      V            BEMF                                              (                  Equation          ⁢                                          ⁢          1                )                            where: ω is the angular velocity of the voice coil; KT is a torque constant; and VBEMF is the back electromagnetic field voltage drop.        
Further, the VBEMF can be determined using the following equation:VBEMF=Vcoil−IcoilRcoil−L di/dt  (Equation 2)                where Vcoil is the voltage across the voice coil, Icoil is the current through the voice coil, Rcoil is the resistance of the voice coil, and L di/dt is the voltage across the coil due to a change in current. Combining the above formulas gives:        
                    ω        =                              1                          K              T                                ⁢                                    (                                                V                  coil                                -                                                      I                    coil                                    ⁢                                      R                    coil                                                  -                                  L                  ⁢                                                            ⅆ                      i                                                              ⅆ                      t                                                                                  )                        .                                              (                  Equation          ⁢                                          ⁢          3                )            
Thus, Rcoil is necessary to determine the angular velocity of the voice coil. As mentioned above, resistance of a voice coil (i.e., Rcoil) is typically only determined when the actuator arm is urged against a crash stop, which prevents the arm from moving. When the actuator arm is not moving, the voice coil is also not moving, causing the back EMF (i.e., VBEMF) to be zero, and the voltage across the voice coil (i.e., Vcoil) to be entirely due to coil resistance (Rcoil), assuming enough time has passed to allow di/dt to also be zero. In this manner coil resistance has been conventionally measured. However, when the actuator arm is traversing a load/unload ramp, or while over the media, the coil resistance may change due to environmental variations, such as temperature variations. Accordingly, there is a need to more accurately keep track of the coil resistance.